Specifically, though not exclusively, the invention is usefully employed as a dilatometer.
This instrument is for measuring a linear size variation of a sample or test piece when the piece undergoes a temperature change.
The test piece is located inside an oven, generally tubular, and controlled variations of the temperature inside are made.
The variations in size of the test piece on varying the temperature are measured by instruments known as dilatometers, which differ among themselves in terms of both the heating system used and the size-variation measuring system.
Mechanical, electronic or optical methodologies are used to measure the size variations, while the heating systems are almost always electrical systems working by radiation.
In mechanical dilatometers the test piece is placed in contact with a system of levers which amplify any even tiny fluctuation in sizexe2x80x94representing such directly on a sheet of paper with a pen.
In electronic dilatometers the test piece is in contact with a rod made of a refractory material which transfers the size variations to an electronic device that converts the movement into an electric signal by means of a differential transformer. The electric signal is then amplified and transformed into a graph through a processing system.
An optical dilatometer measures size variations through a light beam which is deflected by a mirror connected to the sample piece by a mechanical lever. With a laser beam the measurement can be carried out using Abbe""s method (optical interferometry) whose resolution is equal to the wavelength of the light used.
A recent innovation in the field of dilatometry is that measurements of size variation can now be made without touching the test piece, but simply by observing it with a high-definition television camera. In this way measurements can be made of samples in liquid or semi-liquid state.
In the majority of cases, the test piece in the oven is in contact with a system of measurement which inevitably is subject to deformations which influence the measurement and which, in all cases, is in contact with a support that, as it undergoes size change itself during the measurement operations, must influence the accuracy of the measurements. It is therefore always necessary to calibrate the instrument, which in practice means measuring a piece of known dilation properties, so that deviations from zero produced in the instrument itself can be recorded.
In the case of mechanical or electronic dilatometers, where the test piece is located in a holder made of refractory material and the size variations are read by a rod also made of refractory material, the situation which arises is rather complex, inasmuch as all the elements of the measuring system are subject to thermal dilation. The result of this complex sum of different dilations can cause the measuring system dilation to be of the same order as the dilation of the piece being tested. Naturally the dilation of the measuring system must then be subtracted from the dilation of the test piece, and this operation can be done either manually or automatically. These measuring system calibration operations must be repeated frequently because ageing of the materials leads to a change in their thermal-mechanical characteristics; thus a standard control procedure is needed at fixed intervals.
It often happens that the same material gives different dilation measurements if measured using different instruments, due to the fact that the calibration procedure has not been carried out in exactly the same way.
With non-contact optical dilatometers, too, problems connected with instrument calibration persist. Even if the test piece is never touched by the measuring system, it still has to be put in a holder so that it can be perfectly positioned inside the oven chamber. The holder is subject to heat-dilation which must be measured and subtracted from those of the test piece during the test.
All of the preceding leads to very considerable doubts as to the exactitude of the measurements; extreme caution in carrying out the measurements is required in all cases.
The main aim of the present invention is to obviate the limitations and drawbacks in the prior art.
The invention radically solves the problems connected with dilation of the measuring system and/or the test piece holder, virtually eliminating the need for a clumsy and time-consuming calibration curve.
The invention is even more useful in the measurement of dynamic-type dilation, where the thermal-mechanical behaviour of the test piece is to be calculated under conditions of continually-varying temperature.
With the prior art, it is necessary to calibrate the measuring system much more frequently.
Where the thermal-mechanical behaviour of the piece is to be measured under extreme heat gradient conditions, a calibration curve has to be drawn up each time, with the same heat cycle as for the actual test.
With known dilatometers, heat gradients of from 5 to 20 cxc2x0/min are used. Lower gradients require too much time, while higher gradients do not guarantee repeatability of the test.
Today, however, in the industrial field, much higher gradients are reached, meaning that there is a strong demand for thermal-mechanical testing at high heating gradients.
The new device of the invention enables a thermal-mechanical measuring system to be set up both economically and free of those errors caused by the piece-holding and measuring systems in the prior art; and it is particularly suitable for high-gradient measurements.
The invention is also much less sensitive to vibrations with respect to traditional systems.
A further advantage of the invention is the total absence of moving parts in the measuring system. Once assembled and calibrated it is not subject to wear or deterioration, unlike known-type mechanical or electronic systems, which require high-precision mechanical operations on the moving metal parts, which are subject to wear.
These aims and advantages and others besides are all attained by the invention as it is characterised in the appended claims.
The apparatus comprises: a holder (1) for a test piece (2); at least two optical systems (3 and 4), identifying two optical paths located at a predetermined and known reciprocal distance, which are able to focalize, with a predetermined degree of magnification, images of two ends of the test piece (2); the at least two optical systems (3 and 4) being aligned with the holder (1); at least one viewing and measuring device (7) able to collect the images which are focalized by the at least two optical systems (3 and 4). The apparatus is structured to perform measurement of a size of a test piece (2) while completely eliminating any influence on such measurement on the part of the holder or the measuring system.